









































Abstract: Prosocial behavior is a key component of human interaction, and its evolutionary origins are particularly 
interesting for scientists due to its immediate cost to the actor.  Current literature on primate prosocial behavior 
reflects conflicting or incomplete evidence for varying hypotheses, such as reciprocity, kin theory, cooperative 
breeding, and inequity, leaving the evolutionary foundation for this behavior poorly understood.  This review paper 
explores the various proximate and ultimate factors influencing primate prosocial behavior and synthesizes an 
overarching hypothesis that functions to connect the current fragmented state of the literature. It proposes that 
ecology supports kin selection, leading to an increase in cooperative breeding or reciprocal behaviors, which thus 
increases overall social tolerance. Proximate mechanisms, such as underlying neuronal response and situational 
equity, reinforce these behaviors, leading to the altruistic behavior displayed in some primates today.

Aisthesis      Volume 10,  201947

Altruistic Behavior in Primates

by Alyssa Gargagliano

Introduction
 Helping behavior is particularly puzzling for 
scientists because it involves the action of one 
individual benefitting another, potentially at the 
cost to oneself.  There are several forms of helping 
behavior that are necessary to define in order to 
further understand the phenomenon (see Figure 
1).  Prosocial behavior is used as an overarching 
term for helping behavior, simply defined as 
voluntary behaviors that benefit another (Marshall-
Pescini, Dale, Quervel-Chaumette, & Range, 2016). 
Altruistic behavior, a specific subset of prosocial 
behaviors, refers to actions that benefit another at a 
cost to oneself (Silk, 2012).  Cooperative behaviors 
are a subset of prosocial behaviors that require 
joint action of two or more individuals (Brosnan 
& de Waal, 2002).  Overall, prosocial, altruistic, 
and cooperative behaviors are an exhibition of 
prosociality—the motivational predisposition for 

acting in a manner that promotes social existence 
(Burkart, Fehr, Efferson, & van Schaik, 2007). 
 Altruistic behavior is typically cited as a defining 
characteristic of the human species. Humans exhibit 
exceptional acts of altruism in many facets of life, 
such as feeding and paying for offspring until they 
reach adulthood, caring for non-kin individuals, 
dividing labor, donating to charity, and contributing 
to conservation efforts.  The evolution of altruism 
garners particular attention since it is hard to 
understand from a fitness standpoint, given its 
immediate cost to the actor.  An investigation of 
altruistic behavior in humanity’s closest relatives 
across the primate order will hopefully shed some 
light on the behavior demonstrated in ourselves.
 Non-human primates demonstrate altruistic 
behavior in the wild, however, to a lesser extent 
than that exhibited by humans. Evidence shows 
most primates frequently engage in allogrooming, 
many demonstrate mutual group support (such 
as territory patrol and defense from predators), 
some share food provisions, and some demonstrate 
allomaternal care, in which individuals who are 
not the breeding parents of an infant help raise 
the infant (Goosen, 1981; Koenig, 2017; Muller & 
Mitani, 2005). However, these behaviors are not 
uniform across the order, nor consistent from the 
wild to the laboratory. For example, human’s closest 



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living relative, the chimpanzee (Pan troglodytes) 
demonstrates cooperative behaviors in the wild 
and captivity, however, more often than not fails to 
demonstrate prosocial behavior in laboratory setups 
(Silk & House, 2011).  By contrast, the more distantly 
related common marmoset (Callithrix jacchus) 
exhibits consistent prosocial preference both in 
the wild and in laboratory experiments (Burkart et 
al., 2007).  Such interspecific differences have been 
used to propose several hypotheses for the basis 
of altruistic behavior, which are broken down into 
proximate and ultimate influences. 
 Proximate influences immediately influence 
an outcome behavior, such as underlying cognitive 
mechanisms or psychological traits and motivations 
(Marshall-Pescini et al., 2016).  One proximate 
influence referenced in the literature is empathy, 
defined as the capacity to be affected by and share 
the emotional state observed in another (de Waal, 
Leimgruber, & Greenberg, 2008).  It is hypothesized 
elicited empathy in a situation increases incidences 
of prosocial behavior in response.  Another influence 
considered in the literature is increased social 
tolerance, which is the level to which individuals 
behave civilly towards conspecifics. Individuals with 
high social tolerance placed in a given moment are 
expected to be more likely to behave prosocially.  
A third quality that seems to immediately affect 
prosocial tendencies is inequity, characterized 
by unequal reward distributions between two 
individuals in which one receives more of an item 
or an item of greater worth than another individual.  
It is believed in an unequal situation in which the 
recipient would receive a greater reward than 
the donor, there is a decrease in the likelihood of 
prosocial action by the donor (Silk & House, 2011). 
 Comparatively, ultimate influences explain why 
a trait evolved.  These factors increase fitness in the 
long term.  Kin selection is studied as an ultimate 
influence for altruistic behavior because performing 
helping behaviors towards those related to the helper 
will increase the fitness of those with the individual’s 
genes, thus increasing the overall likelihood of the 
perpetuation of the helper’s genes (Marshall-Pescini 
et al., 2016).  Reciprocity also functions as an ultimate 
influence, where altruistic behaviors towards those 
individuals likely to reciprocate will increase fitness 
of the donor in the long term (Jaeggi & Gurven, 
2013). 

 The present review will evaluate the literature 
on altruistic behavior based upon the proximate 
and ultimate influences believed to play a role in its 
exhibition.  Observation of behavior in the wild will 
be considered; however, the paper’s focus will be on 
controlled laboratory studies which truly address the 
motivation behind a prosocial behavior independent 
of confounding factors in the wild. 

Proximate Influences
Mechanisms Behind Empathy
 One mechanism underlying empathy in 
both humans and non-human primates is “state-
matching,” which elicits similar emotions in an 
observer to those displayed by another.   This 
mechanism was first discovered in macaques  
(Macaca mulatta) when researchers observed that 
the same neurons fire within a specific area of the 
premotor cortex both when the monkey performs 
an action and when another individual performs a 
similar action (Rizzolatti, Fadiga, Gallese, & Fogassi, 
1996). Neurons of this nature are termed “mirror 
neurons,” and evidence for mirror neurons has since 
been shown in numerous human studies as well.  An 
experiment utilizing functional magnetic resonance 
imaging in which individuals either observed or 
emulated a facial expression showed similar activity 
in both scenarios within particular structures of 
the brain associated with emotion (Carr, Iacoboni, 
Dubeau, Mazziotta, & Lenzi, 2003).  
 Studies show oxytocin plays an important role in 
empathy as well.  A study investigated oxytocin’s role 
in helping behavior utilizing the Dictator Game—a 
behavioral economic game in which a “proposer” 
subject has the opportunity to allocate any 
proportion of an endowment to a recipient (Camerer 
& Thaler, 1995).  Endowments are in the form of 
cash or currency that can be exchanged for cash, 
and standard setup involves this exchange occurring 
anonymously in order to control for any reputational 
outcomes for a proposer or for reciprocity effects. In 
a Dictator Game with an experimental group that 
received 40 IU oxytocin, results showed individuals 
with increased oxytocin are more likely to allocate 
money more generously (Zak, Stanton, & Ahmadi, 
2007).  Those in the oxytocin group were 80% 
more generous, and thus altruistic, than those 
in the placebo group (Zak et al., 2007).  Several 



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other studies utilizing similar experimental design 
corroborate the role of both oxytocin and mirror 
neurons as biological underpinnings for empathy 
and thus prosocial behavior (Barraza & Zak, 2009; 
Batson, 2014; De Waal, 2007; de Waal & Suchak, 
2010; Jazayari, Ballesta, & Duhamel, 2017; Piliavin & 
Charng, 1990; Schino & Aureli, 2009).

Social Tolerance and Cooperative Breeding
 Social tolerance is increased in species who 
follow a cooperative breeding mechanism, which in 
turn increases the incidence of prosocial behavior 
within those species.  Social tolerance is a trait seen in 
varying levels within species, exhibited by behaviors 
such as territory sharing, food sharing, or reception 
of those of different beliefs or race in humans.  
Cooperative breeding, also known as allomaternal 
care, is a mechanism in which individuals who are 
not the breeding parents of an infant help raise 
the infant.  Cooperative breeding depends upon 
behaviors characteristic of social tolerance, such 
as food sharing and infant assistance, in order 
to function.  As expected, evidence shows social 
tolerance is higher in cooperatively breeding species 
(Burkart & van Schaik, 2010).  Social tolerance has 
since been hypothesized as a proximate mechanism 
for the basis of prosocial behavior (Marshall-Pescini 
et al., 2016).  In order to investigate this claim, 
scientists have turned to laboratory setups testing for 
prosociality in both cooperatively breeding and non-
cooperatively breeding primates, since comparing 
the results between these two groups is a means to 
examine the influence of social tolerance. 
 Cooperatively breeding species consistently 
demonstrate prosocial tendencies when tested in 
the paradigm known as the Prosocial Choice Test 
(PCT) (Figure 2).  The PCT follows a general setup 
in which a donor chooses between two options: a 0/1 
prosocial option that delivers a reward to a recipient 
and nothing to the donor, and a 0/0 neutral option 
that delivers no rewards.  This reward distribution 
is recommended over a 1/1 and 0/1 test—in which 
both the donor and recipient receive rewards in 
the 1/1 distribution and the donor alone receives a 
reward in the 0/1 distribution—because  primates 
demonstrate a preference for larger numbers of 
rewards, which would be a confounding drive for 
an individual to choose the 1/1 distribution (Uher 

& Call, 2008).  A nonsocial control condition, with 
no neighboring recipient, ensures prosocial choices 
are due to the presence of a conspecific.  Options are 
typically selected by the donor using a token system 
or by pulling a tray containing the rewards into reach 
of the recipient.  The prosocial behavior of common 
marmosets, a cooperatively breeding species who 
thus have increased social tolerance, was studied 
utilizing a tray-pulling setup of the PCT (Burkart et 
al., 2007).  Displayed in Figure 3, the proportion of 
prosocial choices of the donors in the test condition 
was significantly greater than the proportion in the 
control condition.  Thus, this cooperatively breeding 
species demonstrated prosociality.  Further analysis 
was performed by partitioning the subject pool by 



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sex and social role.  It was discovered that female 
and male breeders as well as male helpers made the 
prosocial choice preferentially, but female helpers 
showed no preference.  These results are consistent 
with naturalistic observations demonstrating female 
helpers have a minimized role in allomaternal care, 
rarely engaging in infant carrying, which suggests 
a lower social tolerance (Burkart et al., 2007).  The 
researchers of this study were particularly deliberate 
in their methodology and analysis, considering—
and rejecting—the following alternative explanation 
proposed by critics: the subjects do not truly 
understand their role in the experiment and the 
consequences of their choices in terms of who 
benefits, thus discounting prosociality as a motivation 
for their behavior.  However, the study included 
both a baseline pre-test and post-test in which the 
donors had access to the neighboring cage and 
thus could retrieve the food reward.  These baseline 
tests demonstrated a significant preference for the 
prosocial choice when the monkey could collect the 
reward compared to the nonsocial control condition 
in which they could not access the neighboring cage 
and there was no recipient (Burkart et al., 2007).  
These results support the monkeys’ grasp of the 
consequences of their actions.  
 Similar significant results for a prosocial tendency 
have been demonstrated in another cooperatively 
breeding species, the cotton top tamarin (Saguinus 
oedipus) (Cronin, Schroeder, & Snowdon, 2010; 
Hauser, Chen, Chen, & Chuang, 2003).  Furthermore, 
capuchin monkeys (Cebus apella), which follow an 
intermediate degree of cooperative breeding, also 
show a preference for the prosocial option in the 
laboratory (Burkart & van Schaik, 2010; de Waal et 
al., 2008).  This intermediate breeding system likely 
requires an increased social tolerance, though lesser 
than that in fully cooperatively breeding species. 
 Across the literature, non-cooperative breeders 
(or independent breeders) typically demonstrate a 
lack of prosociality in laboratory experiments.  The 
chimpanzee shows no preference for the prosocial 
option when studied utilizing various methodologies 
(Brosnan et al., 2009; Jensen, Hare, Call, & Tomasello, 
2006; Vonk et al., 2008; Yamamoto & Tanaka, 
2010).  Notably, Horner, Carter, Suchak, and de 
Waal (2011) demonstrated a significant preference 
for the prosocial option amongst chimpanzees 

utilizing a setup of the PCT with token exchange.  
The researchers posit this discrepancy is due to prior 
flaws in methodology.  Horner et al. (2011) placed 
chimpanzees in close proximity (<1m) without glass 
barriers and attempted to eliminate any location 
biases that could form when allowing the subject to 
explore both the donor and recipient compartments.  
However, the study by Brosnan et al. (2009), which 
failed to demonstrate a prosocial tendency, did not 
include a glass barrier, nor conditioning that would 
lead towards location biases.  Furthermore, the 
chimpanzees’ understanding of the experimental 
setup was ensured prior to testing (Brosnan et al., 
2009).  In the study by Jensen et al. (2006) which 
also exhibited no prosocial preference, a second 
experimental setup eliminated any possible location 
biases and ensured experimental understanding, and 
although the chimpanzees were in separate rooms, 
the recipients were clearly visible and recognized by 
the donors.  However, following this methodology, 
it is possible results were due to an aversion towards 
inequity.

Inequity
 Studies show primates are inequity averse, and 
thus less likely to engage in an altruistic act when 
the act results in an unequal distribution of rewards.  
Inequity comes in two forms: disadvantageous 
inequity, in which a partner receives a greater reward 
than a focus individual, and advantageous inequity, in 
which the focus individual receives a greater reward 
than his or her partner.  This inequity aversion (IA) 
has structural roots in the brains of both human and 
nonhuman primates.  Humans with lesions on the 
ventromedial and orbitofrontal subdivisions of the 
prefrontal cortex, which evidence shows play a role in 
social behavior, demonstrate an abnormal, decreased 
aversion to social inequity (Jazayari et al., 2017).  
This unusual behavior suggests the lesions disrupt 
the normal human processing of reward outcomes.  
A study on macaque monkeys demonstrated ventral 
striatal neurons were sensitive to reward inequity; 
specific striatal neurons fired during advantageous 
inequity, and another subset of striatal neurons fired 
during disadvantageous inequity (Báez-Mendoza, 
van Coeverden, & Schultz, 2016).  This evidence 
demonstrates monkeys recognize differing reward 



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distributions at the neuronal level, and several 
studies investigate the behavioral consequences of 
these distributions. 
 For nonhuman primates, inequity aversion 
is studied in the laboratory utilizing a modified 
token PCT in which one token delivers a reward 
to the donor (the “selfish choice”), and a different 
token delivers a reward to both individuals but the 
reward to the recipient is of greater value than that 
to the donor (the “prosocial choice”).  Following this 
methodology, significant results showed capuchin 
monkeys were more inclined to act prosocially 
when presented with equal rewards compared to 
conditions containing inequity (Figure 4; de Waal 
et al., 2008).  Preference of the prosocial option 
in the inequity conditions did not significantly 
exceed chance, unlike the equal rewards conditions.  
However, prosocial choice was significantly greater 
in the inequity conditions compared to the control 
condition in which the donor was unable to see the 
neighboring recipient conspecific.  
 Inequity aversion is also studied utilizing a test 
paradigm in which an individual is given a token that 
can be exchanged for a reward, and depending on 
the token, it is exchanged for either a less desirable or 
equally desirable reward compared to that given to a 
neighboring conspecific.  The number of exchange 
refusals, in which the subject either refuses to give 
the token in exchange for the reward or the subject 
refuses the reward upon exchange, are measured per 
trial.  Utilizing this methodology, researchers found 
chimpanzees demonstrated a significantly greater 
percentage of exchange refusals when receiving a 
reward of less desirable quality, thus demonstrating 
an aversion to inequity (Figure 5; Brosnan, Schiff, & 
de Waal, 2005).  However, upon replication of this 
study, researchers failed to reproduce these findings 
in chimpanzees, bonobos, and orangutans, rather 

finding no significant aversion to inequity (see Figure 
6; Bräuer, Call, & Tomasello, 2009).  Bräuer et al. 
(2009) made a slight procedural change in order to 
reduce confounds from the methodology by Brosnan 
et al. (2005), which may explain the difference in 
results. Brosnan et al. (2005) explicitly held up 
the more favorable food (grapes) to the primates 
before each control trial, which may have created an 
expectation that the subjects would receive a grape 
during the control, leading them to be more likely 
to exchange during the control compared to the test 
trials.  While replicating, Bräuer et al. (2009) did not 
explicitly hold up the grape each time, although all 
rewards were visible at all times. 
 Mixed results for primate inequity aversion may 
be affected by closeness of the recipient.  In humans, 
results of economic behavioral games such as the 
Dictator Game suggest individuals have a strong 
preference for equitable outcomes (Silk & House, 
2011).  However, humans tend to respond differently 
to inequity when the other recipient is closely related.  
Each of the nonhuman primate studies mentioned 
in this section found the same trend, where level of 
relation to the partner appeared to play a slight role 
in the number of prosocial choices by the donor.  
This confounding factor is known as kin selection 
and is discussed in detail in the next section. 

Ultimate Influences
Kin Selection
 Kin selection is studied in the laboratory using 
PCT paradigms in which the recipients vary in 
relatedness to the donors.  If the kin selection 
hypothesis is correct, results should indicate 



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significant prosocial choice for recipients who are 
related to the donor and no bias for the prosocial 
choice in trials with a stranger.
 The role of kin selection on capuchin 
monkey prosocial behavior was investigated by 
performing multiple PCT trials with recipients of 
varying relatedness (de Waal et al., 2008).  Results 
demonstrated prosocial tendencies increased with 
social closeness, being lowest towards strangers and 
highest towards kin (see Figure 4).  All test conditions 
with equal rewards resulted in a significant preference 
for the prosocial option, greater than that due to 
chance, except for the trial with strangers (de Waal 
et al., 2008).  Similarly, when studying the influence 
of inequity on chimpanzees, results showed a 
significant correlation to relatedness (Brosnan et al., 
2005). Measuring the amount of exchange refusals 
during trials with unequal rewards, the researchers 
found individuals from a long-term social group 
(housed together for more than 30 years) almost 
never refused an exchange, while those from short-
term or unelated groups showed significant aversion 
to disadvantageous inequity with a much greater 
proportion of refusals (Figure 5; Brosnan et al., 
2005). 
 However, studies honing in on the motivation 
behind prosocial behavior fail to support the 
kin selection hypothesis.  In a study on common 
marmosets, individuals were placed in a PCT 
and found to show a significant preference for the 
prosocial option when a recipient was present 
compared to the nonsocial control (Figure 3; 
Burkart et al., 2007).  In order to eliminate the 
possibility that the results were due to kin selection, 
the researchers performed the trials again but with 
dyads of unrelated individuals.  Results once again 
showed donors pulled the prosocial tray significantly 
more often with a partner present than when alone 
(Burkart et al., 2007). Thus, donors show equally 
strong unsolicited prosocial behavior towards 
nonkin as kin.  The previously mentioned study on 
chimpanzees, with results indicating a significant 
preference for the prosocial option utilizing the 
PCT, also showed no significant difference between 
kin and nonkin pairs (Horner et al., 2011).  Results 
collectively indicate level of relatedness may play a 
role in prosocial behavior, but prosocial behavior is 
not contingent upon kinship. 

Reciprocity
 Altruistic behaviors may depend on the 
likelihood of future reciprocity. Long-term 
observation of individuals in group living in the wild 
and captivity (emulating natural circumstances) is 
often used to study reciprocity, as future reciprocal 
behaviors can be directly observed.  Evidence in these 
observational studies suggests individuals direct 
their altruistic behaviors towards those conspecifics 
that reciprocate most (Schino & Aureli, 2009).  A 
review paper investigated the effect of receiving 
food and/or other commodities on giving food in 32 
independent study populations (eight monkey, eight 
ape, and 16 human populations) (Jaeggi & Gurven, 
2013).  Results indicated an overall weighted effect 
size significantly greater than zero for each collective 
group—monkey, ape, and human—demonstrating 
greater likelihood of giving food after receiving food.  
Additionally, there were no significant differences 
in effect sizes between each group  (Jaeggi & 
Gurven, 2013).  These results indicate a significant 
independent contribution of reciprocity to primate 
helping behavior across the order.  However, in 
observational studies, there is high possibility for 
confounding variables; laboratory studies are often 
used to hone in on the direct motivation behind 
prosocial behavior in a controlled setting.
 A study investigating unsolicited prosociality 
using the PCT specifically controlled for reciprocity 
by eliminating the possibility of reciprocity in its 
design (Burkart et al., 2007).  Results showed a 
significant preference for the prosocial option when 
paired with a conspecific compared to the nonsocial 
control (Figure 3).  Dyads were tested in only one 
direction, the donor never begged to receive food in 
return, no reciprocal exchanges were ever observed, 
and calculated reciprocity never produced false 
positive results in other species known for reciprocity 
during the prosocial test (Burkart et al., 2007).  
Thus, the results of a prosocial preference were 
independent of reciprocity.  Numerous studies on 
various different species, such as chimpanzees and 
cottontop tamarins, indicate the same insignificant 
results when studying the influence of reciprocity on 
prosocial choices in the laboratory setting (Cronin 
et al., 2010; Horner et al., 2011).  Because reciprocity 
seems to play a role in the wild but is not a necessity 



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Aisthesis      Volume 10,  2019

for prosocial behavior in the laboratory, it seems it 
may increase the likelihood of prosocial behavior 
but is nonessential. 

Conclusion
Summary of Results
 After assessing the literature in light of various 
hypotheses thought to play a role in primate 
prosocial behavior, several general results surface.  
In terms of the proximate mechanisms, evidence 
shows mirror neurons and oxytocin underlie 
empathy in both human and non-human primates.  
Furthermore, increased social tolerance from 
cooperative breeding shows a strong correlation to 
increased prosocial behavior. However, there are 
some inconsistencies with chimpanzees, who are a 
non-cooperative breeding species and sometimes 
demonstrate prosocial behavior in the laboratory, 
though for the most part they do not.  Finally, there 
is a general trend of aversion to inequity, but it does 
not prevent prosociality.  In terms of the ultimate 
mechanisms, laboratory results indicate that neither 
kinship nor reciprocity are necessary for the presence 
of prosociality.  However, there seems to be a trend 
in relatedness increasing prosocial behavior for kin 
theory and evidence for reciprocity in the wild. 
      
Proposed Framework for Prosociality
 Based upon these results, these various 
proximate and ultimate factors may be associated 
in the manner depicted in Figure 7.  At some point 
in history, there is some specific ecology that favors 
kin selection in order to increase general fitness of 
an individual’s genes. These pressures then lead to 
a general system of behavior, whether cooperative 
breeding or reciprocity, that helps those closely 
related to or associated with the helper.  This suite 
of behaviors favors the increase of certain traits 
within the individuals in order to maximize the 
functionality of the suite of behaviors.  This includes 
an increased social tolerance.  By increasing social 
tolerance, it is easier then to carry out these species’ 
behavioral structures. This general increase in social 
tolerance may then lead to prosocial behavior in 
other contexts, outside of cooperative breeding and 
reciprocity. Because cooperative breeding shows the 
strongest and most widespread relationship in the 
laboratory, it is most likely the strongest influence on 

prosocial behavior.  Cooperative breeding seems to 
draw a clear connection to increased social tolerance, 
whereas this is not clear for reciprocity.  Since kin 
selection and reciprocity show a trend but rarely any 
significant results, they are most likely contributing 
factors but not necessary in order to demonstrate 
prosocial behavior. 
 The few outlier cases with chimpanzees may be 
due to an interaction of the other influence factors, 
like reciprocity, combined with the chimpanzee’s 
higher level of cognitive ability compared to the 
cooperatively breeding species of monkeys.  It is 
possible this increased cognitive ability enables 
the chimpanzee to behave more variably, based on 
learned behaviors and external situational factors, 
whereas those of lesser cognitive ability follow 
a certain set of innate behaviors in response to 
stimulus.  Furthermore, it is possible an ancestor 
of the chimpanzee was once a cooperative breeder, 
increasing the social tolerance within the species, 
but since then, this ancestral structure is no longer 
necessary. This may offer one explanation as to why 
the increased social tolerance trait is retained within 
the chimpanzee to some extent. 
 Outside characteristics of the situation also 
seem to be contributing factors towards prosociality. 
Given the mirror neurons within the brain, certain 
situations elicit varying emotion within an individual, 
which may also influence the likelihood of a prosocial 
behavior. Again, the increased cognitive ability of 
chimpanzees may also increase the weight of this 
empathetic neuronal response, perhaps influencing 
their behavior more strongly compared to monkeys.  

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An additional situational influence is the existence 
of inequity.  Inequity shows a potential trend, but the 
existence of inequity does not prohibit prosociality, 
making it only a contributing factor. 
 Overall, prosociality, and thus altruism, seem to 
be contingent upon the existence of increased social 
tolerance and the general mechanisms of empathy.  
This increased social tolerance is most likely due to 
cooperative breeding, whether the social structure 
still stands in the species or fell out of favor at some 
point in evolution.  If not cooperative breeding, it 
must be some third, unknown structure that is yet 
observed and studied.  The rest of the factors are 
likely contributing but not necessary towards the 
existence of prosociality. 
 Based upon this framework for prosociality, 
there are a few suggestions for future directions of 
research.  It would be valuable to study other taxa, 
aside from the primates, in order to see whether 
other cooperative breeding species also demonstrate 
prosociality.  If the trend transcends the primate 
order, it would further support this hypothesis.  
Hand in hand with the study of cooperative 
breeding in other taxa, it should also be explored 
whether the individuals have mirror neurons.  If the 
emergence of this neuronal structure coincides with 
the emergence of prosocial behavior, that also will 
strengthen the neuronal connection to prosociality. 
In all, an in-depth phylogenetic analysis is necessary 
to truly discover what led to human altruism. 
      
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